Mycobacterial infections often manifest as diseases such as tuberculosis. Human infections caused by mycobacteria have been widespread since ancient times, and tuberculosis remains a leading cause of death today. Although the incidence of the disease declined, in parallel with advancing standards of living, since the mid-nineteenth century, mycobacterial diseases still constitute a leading cause of morbidity and mortality in countries with limited medical resources. Additionally, mycobacterial diseases can cause overwhelming, disseminated disease in immunocompromised patients. In spite of the efforts of numerous health organizations worldwide, the eradication of mycobacterial diseases has never been achieved, nor is eradication imminent. Nearly one third of the world's population is infected with mycobacterium tuberculosis complex, commonly referred to as tuberculosis (TB), with approximately 8 million new cases, and two to three million deaths attributable to TB yearly. Tuberculosis (TB) is the cause of the largest number of human deaths attributable to a single etiologic agent (see Dye et al., J. Am. Med. Association, 282, 677-686, (1999); and 2000 WHO/OMS Press Release).
After decades of decline, TB is now on the rise. In the United States, up to 10 million individuals are believed to be infected. Almost 28,000 new cases were reported in 1990, constituting a 9.4 percent increase over 1989. A sixteen percent increase in TB cases was observed from 1985 to 1990. Overcrowded living conditions and shared air spaces are especially conducive to the spread of TB, contributing to the increase in instances that have been observed among prison inmates, and among the homeless in larger U.S. cities. Approximately half of all patients with “Acquired Immune Deficiency Syndrome” (AIDS) will acquire a mycobacterial infection, with TB being an especially devastating complication. AIDS patients are at higher risks of developing clinical TB, and anti-TB treatment seems to be less effective than in non-AIDS patients. Consequently, the infection often progresses to a fatal disseminated disease.
Mycobacteria other than M. tuberculosis are increasingly found in opportunistic infections that plague the AIDS patient. Organisms from the M. avium-intracellulare complex (MAC), especially serotypes four and eight, account for 68% of the mycobacterial isolates from AIDS patients. Enormous numbers of MAC are found (up to 1010 acid-fast bacilli per gram of tissue), and consequently, the prognosis for the infected AIDS patient is poor.
The World Health Organization (WHO) continues to encourage the battle against TB, recommending prevention initiatives such as the “Expanded Program on Immunization” (EPI), and therapeutic compliance initiatives such as “Directly Observed Treatment Short-Course” (DOTS). For the eradication of TB, diagnosis, treatment, and prevention are equally important. Rapid detection of active TB patients will lead to early treatment by which about 90% cure is expected. Therefore, early diagnosis is critical for the battle against TB. In addition, therapeutic compliance will ensure not only elimination of infection, but also reduction in the emergence of drug-resistance strains.
The emergence of drug-resistant M. tuberculosis is an extremely disturbing phenomenon. The rate of new TB cases proven resistant to at least one standard drug increased from 10 percent in the early 1980's to 23 percent in 1991. Compliance with therapeutic regimens, therefore, is also a crucial component in efforts to eliminate TB and prevent the emergence of drug resistant strains. Equally important is the development of new therapeutic agents that are effective as vaccines, and as treatments, for disease caused by drug resistant strains of mycobacteria.
Although over 37 species of mycobacteria have been identified, more than 95% of all human infections are caused by six species of mycobacteria: M. tuberculosis, M. avium intracellulare, M. kansasii, M. fortuitum, M. chelonae, and M. leprae. The most prevalent mycobacterial disease in humans is tuberculosis (TB) which is predominantly caused by mycobacterial species comprising M. tuberculosis, M. bovis, or M. africanum (Merck Manual 1992). Infection is typically initiated by the inhalation of infectious particles which are able to reach the terminal pathways in lungs. Following engulfment by alveolar macrophages, the bacilli are able to replicate freely, with eventual destruction of the phagocytic cells. A cascade effect ensues wherein destruction of the phagocytic cells causes additional macrophages and lymphocytes to migrate to the site of infection, where they too are ultimately eliminated. The disease is further disseminated during the initial stages by the infected macrophages which travel to local lymph nodes, as well as into the blood stream and other tissues such as the bone marrow, spleen, kidneys, bone and central nervous system. (See Murray et al. Medical Microbiology, The C.V. Mosby Company 219-230 (1990)).
There is still no clear understanding of the factors which contribute to the virulence of mycobacteria. Many investigators have implicated lipids of the cell wall and bacterial surface as contributors to colony morphology and virulence. Evidence suggests that C-mycosides, on the surface of certain mycobacterial cells, are important in facilitating survival of the organism within macrophages. Trehalose 6,6′ dimycolate, a cord factor, has been implicated for other mycobacteria.
The interrelationship of colony morphology and virulence is particularly pronounced in M. avium. M. avium bacilli occur in several distinct colony forms. Bacilli which grow as transparent, or rough, colonies on conventional laboratory media are multiplicable within macrophages in tissue culture, are virulent when injected into susceptible mice, and are resistant to antibiotics. Rough or transparent bacilli, which are maintained on laboratory culture media, often spontaneously assume an opaque R colony morphology, at which time they are not multiplicable in macrophages, are avirulent in mice, and are highly susceptible to antibiotics. The differences in colony morphology between the transparent, rough and opaque strains of M. avium are almost certainly due to the presence of a glycolipid coating on the surface of transparent and rough organisms which acts as a protective capsule. This capsule, or coating, is composed primarily of C-mycosides which apparently shield the virulent M. avium organisms from lysosomal enzymes and antibiotics. By contrast, the non-virulent opaque forms of M. avium have very little C-mycoside on their surface. Both the resistance to antibiotics and the resistance to killing by macrophages have been attributed to the glycolipid barrier on the surface of M. avium. 
Diagnosis of mycobacterial infection is confirmed by the isolation and identification of the pathogen, although conventional diagnosis is based on sputum smears, chest X-ray examination (CXR), and clinical symptoms. Isolation of mycobacteria on a medium takes as long as four to eight weeks. Species identification takes a further two weeks. There are several other techniques for detecting mycobacteria such as the polymerase chain reaction (PCR), mycobacterium tuberculosis direct test, or amplified mycobacterium tuberculosis direct test (MTD), and detection assays that utilize radioactive labels.
One diagnostic test that is widely used for detecting infections caused by M. tuberculosis is the tuberculin skin test. Although numerous versions of the skin test are available, typically one of two preparations of tuberculin antigens are used: old tuberculin (OT), or purified protein derivative (PPD). The antigen preparation is either injected into the skin intradermally, or is topically applied and is then invasively transported into the skin with the use of a multiprong inoculator (Tine test). Several problems exist with the skin test diagnosis method. For example, the Tine test is not generally recommended because the amount of antigen injected into the intradermal layer cannot be accurately controlled. (See Murray et al. Medical Microbiology, The C.V. Mosby Company 219-230 (1990)).
Although the tuberculin skin tests are widely used, they typically require two to three days to generate results, and many times, the results are inaccurate since false positives are sometimes seen in subjects who have been exposed to mycobacteria, but are healthy. In addition, instances of mis-diagnosis are frequent since a positive result is observed not only in active TB patients, but also in persons vaccinated with Bacille Calmette-Guerin (BCG), and those who had been infected with mycobacteria, but have not developed the disease. It is hard therefore, to distinguish active TB patients from the others, such as household TB contacts, by the tuberculin skin test. Additionally, the tuberculin test often produces a cross-reaction in those individuals who were infected with mycobacteria other than M. tuberculosis (MOTT). Therefore, diagnosis using the skin tests currently available is frequently subject to error and inaccuracies.
The standard treatment for tuberculosis caused by drug-sensitive organisms is a six-month regimen consisting of four drugs given for two months, followed by two drugs given for four months. The two most important drugs, given throughout the six-month course of therapy, are isoniazid and rifampin. Although the regimen is relatively simple, its administration is quite complicated. Daily ingestion of eight or nine pills is often required during the first phase of therapy; a daunting and confusing prospect. Even severely ill patients are often symptom free within a few weeks, and nearly all appear to be cured within a few months. If the treatment is not continued to completion, however, the patient may experience a relapse, and the relapse rate for patients who do not continue treatment to completion is high. A variety of forms of patient-centered care are used to promote adherence with therapy. The most effective way of ensuring that patients are taking their medication is to use directly observed therapy, which involves having a member of the health care team observe the patient take each dose of each drug. Directly observed therapy can be provided in the clinic, the patient's residence, or any mutually agreed upon site. Nearly all patients who have tuberculosis caused by drug-sensitive organisms, and who complete therapy will be cured, and the risk of relapse is very low (“Ending Neglect: The Elimination of Tuberculosis in the United States” ed. L. Geiter Committee on the Elimination of Tuberculosis in the United States Division of Health Promotion and Disease Prevention, Institute of Medicine. Unpublished.)
What is needed are effective therapeutic regimens that include improved vaccination and treatment protocols. Currently available therapeutics are no longer consistently effective as a result of the problems with treatment compliance, and these compliance problems contribute to the development of drug resistant mycobacterial strains.
Ethambutol (EMB) is a widely used antibiotic for the treatment of TB, with over 300 million doses delivered for tuberculosis therapy in 1988.

Ethambutol, developed by Lederle Laboratories in the 1950s, has low toxicity and is a good pharmacokinetic. However, ethambutol has a high Minimum Inhibition Concentration (MIC) of about 5 μg/ml, and can cause optic neuritis. Thus, there is an increasing need for new, and more effective, therapeutic compositions (See for example, U.S. Pat. Nos. 3,176,040, 4,262,122; 4,006,234; 3,931,157; 3,931,152; U.S. Re. 29,358; and Häusler et al., Bioorganic & Medicinal Chemistry Letters 11 (2001) 1679-1681). In the decoder years since the discovery of the beneficial effects of ethambutol, few pharmacological advances in TB treatment have been developed. Moreover, with the combined emergence of drug resistant strains, and the more prevalent spread of mycobacterial disease, it is becoming seriously apparent that new therapeutic compositions are crucial in the fight against tuberculosis.
Clearly effective therapeutic regimens that include improved vaccination and treatment protocols are needed. A therapeutic vaccine that would prevent the onset of tuberculosis, and therefore eliminate the need for therapy is desirable. Although currently available therapeutics such as ethambutol are effective, the emergence of drug resistant strains has necessitated new formulations and compositions that are more versatile than ethambutol. What is needed are new anti-tubercular drugs that provide highly effective treatment, and shorten and simplify tuberculosis chemotherapy.